Re: [Xen-devel] [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor

[Gleb Natapov <gleb@xxxxxxxxxx>]

On Mon, Jun 24, 2013 at 06:13:42PM +0530, Raghavendra K T wrote:
> kvm : Paravirtual ticketlocks support for linux guests running on KVM 
> hypervisor
> 
> From: Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>
> 
> During smp_boot_cpus  paravirtualied KVM guest detects if the hypervisor has
> required feature (KVM_FEATURE_PV_UNHALT) to support pv-ticketlocks. If so,
>  support for pv-ticketlocks is registered via pv_lock_ops.
> 
> Use KVM_HC_KICK_CPU hypercall to wakeup waiting/halted vcpu.
> 
> Signed-off-by: Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Suzuki Poulose <suzuki@xxxxxxxxxx>
> [Raghu: check_zero race fix, enum for kvm_contention_stat
> jumplabel related changes ]
> Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_para.h |   14 ++
>  arch/x86/kernel/kvm.c           |  255 
> +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 267 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
> index 695399f..427afcb 100644
> --- a/arch/x86/include/asm/kvm_para.h
> +++ b/arch/x86/include/asm/kvm_para.h
> @@ -118,10 +118,20 @@ void kvm_async_pf_task_wait(u32 token);
>  void kvm_async_pf_task_wake(u32 token);
>  u32 kvm_read_and_reset_pf_reason(void);
>  extern void kvm_disable_steal_time(void);
> -#else
> -#define kvm_guest_init() do { } while (0)
> +
> +#ifdef CONFIG_PARAVIRT_SPINLOCKS
> +void __init kvm_spinlock_init(void);
> +#else /* !CONFIG_PARAVIRT_SPINLOCKS */
> +static inline void kvm_spinlock_init(void)
> +{
> +}
> +#endif /* CONFIG_PARAVIRT_SPINLOCKS */
> +
> +#else /* CONFIG_KVM_GUEST */
> +#define kvm_guest_init() do {} while (0)
>  #define kvm_async_pf_task_wait(T) do {} while(0)
>  #define kvm_async_pf_task_wake(T) do {} while(0)
> +
>  static inline u32 kvm_read_and_reset_pf_reason(void)
>  {
>       return 0;
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index cd6d9a5..97ade5a 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -34,6 +34,7 @@
>  #include <linux/sched.h>
>  #include <linux/slab.h>
>  #include <linux/kprobes.h>
> +#include <linux/debugfs.h>
>  #include <asm/timer.h>
>  #include <asm/cpu.h>
>  #include <asm/traps.h>
> @@ -419,6 +420,7 @@ static void __init kvm_smp_prepare_boot_cpu(void)
>       WARN_ON(kvm_register_clock("primary cpu clock"));
>       kvm_guest_cpu_init();
>       native_smp_prepare_boot_cpu();
> +     kvm_spinlock_init();
>  }
>  
>  static void __cpuinit kvm_guest_cpu_online(void *dummy)
> @@ -523,3 +525,256 @@ static __init int activate_jump_labels(void)
>       return 0;
>  }
>  arch_initcall(activate_jump_labels);
> +
> +/* Kick a cpu by its apicid. Used to wake up a halted vcpu */
> +void kvm_kick_cpu(int cpu)
> +{
> +     int apicid;
> +
> +     apicid = per_cpu(x86_cpu_to_apicid, cpu);
> +     kvm_hypercall1(KVM_HC_KICK_CPU, apicid);
> +}
> +
> +#ifdef CONFIG_PARAVIRT_SPINLOCKS
> +
> +enum kvm_contention_stat {
> +     TAKEN_SLOW,
> +     TAKEN_SLOW_PICKUP,
> +     RELEASED_SLOW,
> +     RELEASED_SLOW_KICKED,
> +     NR_CONTENTION_STATS
> +};
> +
> +#ifdef CONFIG_KVM_DEBUG_FS
> +#define HISTO_BUCKETS        30
> +
> +static struct kvm_spinlock_stats
> +{
> +     u32 contention_stats[NR_CONTENTION_STATS];
> +     u32 histo_spin_blocked[HISTO_BUCKETS+1];
> +     u64 time_blocked;
> +} spinlock_stats;
> +
> +static u8 zero_stats;
> +
> +static inline void check_zero(void)
> +{
> +     u8 ret;
> +     u8 old;
> +
> +     old = ACCESS_ONCE(zero_stats);
> +     if (unlikely(old)) {
> +             ret = cmpxchg(&zero_stats, old, 0);
> +             /* This ensures only one fellow resets the stat */
> +             if (ret == old)
> +                     memset(&spinlock_stats, 0, sizeof(spinlock_stats));
> +     }
> +}
> +
> +static inline void add_stats(enum kvm_contention_stat var, u32 val)
> +{
> +     check_zero();
> +     spinlock_stats.contention_stats[var] += val;
> +}
> +
> +
> +static inline u64 spin_time_start(void)
> +{
> +     return sched_clock();
> +}
> +
> +static void __spin_time_accum(u64 delta, u32 *array)
> +{
> +     unsigned index;
> +
> +     index = ilog2(delta);
> +     check_zero();
> +
> +     if (index < HISTO_BUCKETS)
> +             array[index]++;
> +     else
> +             array[HISTO_BUCKETS]++;
> +}
> +
> +static inline void spin_time_accum_blocked(u64 start)
> +{
> +     u32 delta;
> +
> +     delta = sched_clock() - start;
> +     __spin_time_accum(delta, spinlock_stats.histo_spin_blocked);
> +     spinlock_stats.time_blocked += delta;
> +}
> +
> +static struct dentry *d_spin_debug;
> +static struct dentry *d_kvm_debug;
> +
> +struct dentry *kvm_init_debugfs(void)
> +{
> +     d_kvm_debug = debugfs_create_dir("kvm", NULL);
> +     if (!d_kvm_debug)
> +             printk(KERN_WARNING "Could not create 'kvm' debugfs 
> directory\n");
> +
> +     return d_kvm_debug;
> +}
> +
> +static int __init kvm_spinlock_debugfs(void)
> +{
> +     struct dentry *d_kvm;
> +
> +     d_kvm = kvm_init_debugfs();
> +     if (d_kvm == NULL)
> +             return -ENOMEM;
> +
> +     d_spin_debug = debugfs_create_dir("spinlocks", d_kvm);
> +
> +     debugfs_create_u8("zero_stats", 0644, d_spin_debug, &zero_stats);
> +
> +     debugfs_create_u32("taken_slow", 0444, d_spin_debug,
> +                &spinlock_stats.contention_stats[TAKEN_SLOW]);
> +     debugfs_create_u32("taken_slow_pickup", 0444, d_spin_debug,
> +                &spinlock_stats.contention_stats[TAKEN_SLOW_PICKUP]);
> +
> +     debugfs_create_u32("released_slow", 0444, d_spin_debug,
> +                &spinlock_stats.contention_stats[RELEASED_SLOW]);
> +     debugfs_create_u32("released_slow_kicked", 0444, d_spin_debug,
> +                &spinlock_stats.contention_stats[RELEASED_SLOW_KICKED]);
> +
> +     debugfs_create_u64("time_blocked", 0444, d_spin_debug,
> +                        &spinlock_stats.time_blocked);
> +
> +     debugfs_create_u32_array("histo_blocked", 0444, d_spin_debug,
> +                  spinlock_stats.histo_spin_blocked, HISTO_BUCKETS + 1);
> +
> +     return 0;
> +}
> +fs_initcall(kvm_spinlock_debugfs);
> +#else  /* !CONFIG_KVM_DEBUG_FS */
> +static inline void add_stats(enum kvm_contention_stat var, u32 val)
> +{
> +}
> +
> +static inline u64 spin_time_start(void)
> +{
> +     return 0;
> +}
> +
> +static inline void spin_time_accum_blocked(u64 start)
> +{
> +}
> +#endif  /* CONFIG_KVM_DEBUG_FS */
> +
> +struct kvm_lock_waiting {
> +     struct arch_spinlock *lock;
> +     __ticket_t want;
> +};
> +
> +/* cpus 'waiting' on a spinlock to become available */
> +static cpumask_t waiting_cpus;
> +
> +/* Track spinlock on which a cpu is waiting */
> +static DEFINE_PER_CPU(struct kvm_lock_waiting, lock_waiting);
> +
> +static void kvm_lock_spinning(struct arch_spinlock *lock, __ticket_t want)
> +{
> +     struct kvm_lock_waiting *w;
> +     int cpu;
> +     u64 start;
> +     unsigned long flags;
> +
> +     w = &__get_cpu_var(lock_waiting);
> +     cpu = smp_processor_id();
> +     start = spin_time_start();
> +
> +     /*
> +      * Make sure an interrupt handler can't upset things in a
> +      * partially setup state.
> +      */
> +     local_irq_save(flags);
> +
> +     /*
> +      * The ordering protocol on this is that the "lock" pointer
> +      * may only be set non-NULL if the "want" ticket is correct.
> +      * If we're updating "want", we must first clear "lock".
> +      */
> +     w->lock = NULL;
> +     smp_wmb();
> +     w->want = want;
> +     smp_wmb();
> +     w->lock = lock;
> +
> +     add_stats(TAKEN_SLOW, 1);
> +
> +     /*
> +      * This uses set_bit, which is atomic but we should not rely on its
> +      * reordering gurantees. So barrier is needed after this call.
> +      */
> +     cpumask_set_cpu(cpu, &waiting_cpus);
> +
> +     barrier();
> +
> +     /*
> +      * Mark entry to slowpath before doing the pickup test to make
> +      * sure we don't deadlock with an unlocker.
> +      */
> +     __ticket_enter_slowpath(lock);
> +
> +     /*
> +      * check again make sure it didn't become free while
> +      * we weren't looking.
> +      */
> +     if (ACCESS_ONCE(lock->tickets.head) == want) {
> +             add_stats(TAKEN_SLOW_PICKUP, 1);
> +             goto out;
> +     }
> +
> +     /* Allow interrupts while blocked */
> +     local_irq_restore(flags);
> +
So what happens if an interrupt comes here and an interrupt handler
takes another spinlock that goes into the slow path? As far as I see
lock_waiting will become overwritten and cpu will be cleared from
waiting_cpus bitmap by nested kvm_lock_spinning(), so when halt is
called here after returning from the interrupt handler nobody is going
to wake this lock holder. Next random interrupt will "fix" it, but it
may be several milliseconds away, or never. We should probably check
if interrupt were enabled and call native_safe_halt() here.

> +     /* halt until it's our turn and kicked. */
> +     halt();
> +
> +     local_irq_save(flags);
> +out:
> +     cpumask_clear_cpu(cpu, &waiting_cpus);
> +     w->lock = NULL;
> +     local_irq_restore(flags);
> +     spin_time_accum_blocked(start);
> +}
> +PV_CALLEE_SAVE_REGS_THUNK(kvm_lock_spinning);
> +
> +/* Kick vcpu waiting on @lock->head to reach value @ticket */
> +static void kvm_unlock_kick(struct arch_spinlock *lock, __ticket_t ticket)
> +{
> +     int cpu;
> +
> +     add_stats(RELEASED_SLOW, 1);
> +     for_each_cpu(cpu, &waiting_cpus) {
> +             const struct kvm_lock_waiting *w = &per_cpu(lock_waiting, cpu);
> +             if (ACCESS_ONCE(w->lock) == lock &&
> +                 ACCESS_ONCE(w->want) == ticket) {
> +                     add_stats(RELEASED_SLOW_KICKED, 1);
> +                     kvm_kick_cpu(cpu);
> +                     break;
> +             }
> +     }
> +}
> +
> +/*
> + * Setup pv_lock_ops to exploit KVM_FEATURE_PV_UNHALT if present.
> + */
> +void __init kvm_spinlock_init(void)
> +{
> +     if (!kvm_para_available())
> +             return;
> +     /* Does host kernel support KVM_FEATURE_PV_UNHALT? */
> +     if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
> +             return;
> +
> +     printk(KERN_INFO "KVM setup paravirtual spinlock\n");
> +
> +     static_key_slow_inc(&paravirt_ticketlocks_enabled);
> +
> +     pv_lock_ops.lock_spinning = PV_CALLEE_SAVE(kvm_lock_spinning);
> +     pv_lock_ops.unlock_kick = kvm_unlock_kick;
> +}
> +#endif       /* CONFIG_PARAVIRT_SPINLOCKS */

--
                        Gleb.

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